(1) Field of the Invention
The present invention generally relates to a cordless phone system capable of scrambling communication signals, and more particularly to a cordless phone system capable of scrambling communication signals in which system communication quality can be prevented from deteriorating by scrambling.
(2) Description of Related Art
Recently, cordless phones have been spread. The cordless phones can be tapped by other persons. Thus, to prevent from being tapped, a scrambling technique has been applied to the cordless phones.
Communication signals to be transmitted in radio are scrambled by a scrambler, for example, as shown in FIG. 1A. This scrambler inverts the frequency spectrum of the input signals and outputs inverted signals having an inverted frequency spectrum, so that the input signals are scrambled. This type of scrambling manner is often referred to as a frequency inversion method.
Referring to FIG. 1A, an input communication signal is supplied to a first low pass filter (LPF) 40, and only frequency components, of the input signal, within a range between a higher cut-off frequency f2 (e.g. 3300 Hz) and a lower cut-off frequency f1 (e.g. 300 Hz) pass through the first low pass filter 40. As a result, the frequency spectrum as shown in FIG. 1B is obtained at an output point (A) of the first low pass filter 40. The communication signal output from the first low pass filter 40 is supplied to a modulator 41. The modulator 41 modulates a carrier having a frequency fm based on the communication signal supplied from the first low pass filter 40, so that two frequency spectrums as shown in FIG. 1C are obtained at an output point (B) of the modulator 41. The first frequency spectrum is in a frequency range between (f2-fm) and (f1-fm), and the second frequency spectrum is in a frequency range between (f1+fm) and (f2+fm). The output signal from the modulator 41 is supplied to a second low pass filter 42, and frequency components in only the first frequency spectrum can pass through the second low pass filter 42. As a result, a frequency spectrum as shown in FIG. 1D is obtained at an output point (C) of the second low pass filter 42. That is, this scrambler outputs an inverted signal having a frequency spectrum as shown in FIG. 1D.
The inverted signal output from the scrambler is modulated in accordance with a frequency modulation (FM) process at a radio frequency of a radio channel, and the modulated signal is transmitted, as a radio signal, via the radio channel. The radio signal is demodulated in a receiver, and then the signal having the inverted frequency spectrum is modulated based on the carrier (fm), so that the signal having an original frequency spectrum is restored. Then the communication signal is obtained.
There are various well-known scrambling manners other than the above frequency inversion method, referred to, for example, as a method in which the radio channel is changed during communication.
FIG. 2 shows a conventional cordless phone system capable of scrambling communication signals.
Referring to FIG. 2, a first cordless phone comprises a personal station A and a base station A. The base station A is coupled to a public data network (e.g. an ISDN or a private network). A second cordless phone comprises a personal station B and a base station B. The base station B is also coupled to the public data network, so that the base stations A and B of the first and second cordless phones are coupled to each other via the public data network. In this conventional cordless phone system, a communication signal corresponding to speech of a user is scrambled in the personal station A of the first cordless phone (a), and the scrambled communication signal is transmitted from the personal station A to the base station A via a predetermined radio channel. The scrambled communication signal received by the base station is descrambled so that the original communication signal is restored (b). The communication signal output from the base station A of the first cordless phone is transmitted to the base station B of the second cordless phone via the public data network. The communication signal received by the base station B of the second cordless phone is scrambled again (c), and the scrambled communication signal is transmitted from the base station B to the personal station B via a predetermined radio channel. The scrambled communication signal received by the personal station B of the second cordless phone is descrambled again (d), and the original communication signal is restored. Then the personal station B of the second cordless phone reproduces speech base on the restored communication signal. In a case where the communication signal is transmitted from the personal station B of the second cordless phone, the personal station B scrambles the communication signal (e), the base station B of the second cordless phone descrambles the scrambled communication signal (f), the base station A of the first cordless phone scrambles the received communication signal again (g), and the personal station A of the first cordless phone descrambles the scrambled communication signal again (h), in the same manner as those in the above case where the communication signal is transmitted from the personal station of the first cordless phone.
In the above conventional cordless phone system, the first cordless phone comprising the personal station A and the base station A carries out both a scrambling process and a corresponding descrambling process. The second cordless phone also carries out both a scrambling process and a corresponding descrambling process. Frequency characteristics of communication signals are changed every time the scrambling or descrambling process is performed. Thus, as a large number of scrambling and the descrambling processes are performed in the conventional cordless phone system, communication quality between the personal stations deteriorates in comparison with the communication quality without scrambling.